1. Field of Invention
This invention relates to an optical system for a reducible exposure apparatus, such as a stepper, used for manufacturing semiconductor devices, and more particularly, to a scanning type catadioptric reducible optical system having a magnification of 1/4 to 1/5 and a high resolution in the ultraviolet frequency band.
2. Description of Related Art
In recent years, semiconductor circuit patterns have become more and more minute, and a demand has arisen for a high-resolution exposure apparatus that is capable of printing such minute patterns.
In order to realize a high-resolution exposure apparatus, the wavelength of the light source must be shortened and, at the same time, the numerical aperture (NA) of the optical system must be increased. However, if the wavelength is shortened, the types of optical glass that can be practically used in the exposure apparatus are very limited because of light absorption. In fact, with a wavelength of 300 nm or less, only synthetic quartz and fluorite can be practically used.
Unfortunately, however, the Abbe constants of the synthetic quartz and fluorite are too close to each other to sufficiently compensate for the chromatic aberration in the system. For this reason, if the wavelength of the light source is shortened to 300 nm or less, it becomes difficult for the projection-optical system that comprises only a refracting optical system to satisfactorily correct the chromatic aberration. In addition, the refractivity of fluorite easily changes in response to a temperature change because of its inferior temperature characteristic. Fluorite also has several problems in the lens grinding process. Thus, it is not easy to achieve a high-resolution exposure apparatus only with a refracting optical system.
On the other hand, many attempts have been made to design the projection-optical system using only a reflecting optical system. In this case, the projection-optical system becomes large in size, and the reflecting surfaces must be made aspheric. It is difficult to form an aspheric surface at a high precision.
In view of these drawbacks, it was proposed to combine a reflecting system and a refracting system made of an optical glass durable to the designated wavelength. Also, many techniques for constructing a reducible projection-optical system using a reflecting/refracting, or catadioptric, optical system have been proposed. Among them, many systems form two or more intermediate images in their optical system. Meanwhile, techniques for forming only one intermediate image in the optical system are disclosed in Japanese Patent Publication No. 5-25170, the Japanese Patent Application Laid-open Nos. 63-163319 and 4-234722, and U.S. Pat. No. 4,779,966.
In particular, Japanese Patent Application Laid-open No. 4-234722 and U.S. Pat. No. 4,779,966 disclose optical systems that use only one concave mirror. The concave mirror is used in the double path lens system which comprises only concave lenses, without using convex power lenses. In this structure, the light flux is apt to diverge when it strikes the concave mirror and, therefore, the diameter of the concave mirror inevitably increases.
The double path lens system disclosed in Japanese Application 4-234722 is perfectly symmetric for the purpose of preventing aberrations as much as possible in this lens system, and of reducing the burden of aberration correction on the subsequent optical system. However, it is difficult for the symmetric optical system to obtain an adequate working distance near the first plane and, therefore, a half prism must be used.
The optical system disclosed in U.S. Pat. No. 4,779,966 uses a mirror in the secondary focusing optical system positioned behind the intermediate image. In this arrangement, the light flux can not be narrowed because adequate brightness must be required for the optical system, and the divergent light flux strikes the concave mirror surface. For this reason, it is difficult to make the mirror smaller.
If a plurality of mirrors is used, the number of lenses in the refracting optical system can be reduced. However, this type of system has other potential problems.
One problem is that if this type of catadioptric optical system is used as an objective lens, there is no effective diaphragm position in this catadioptric optical system. That is, recent phase-shift techniques allow the phase of a selected area on the mask to be shifted in order to improve the resolution, while achieving a sufficient amount of focal depth. For further improvement of the resolution, the NA ratio .sigma. of the illumination-optical system to the focusing-optical system is made variable by providing aperture stops in both systems. However, in the above-mentioned multiple-mirror system, an aperture stop can be positioned only in the illumination-optical system, and there is no place for a diaphragm in the catadioptric optical system which serves as the objective lens of the apparatus.
If a multiple-mirror type double path lens system is positioned near the second plane on the reduction side (i.e., on the wafer side) in the catadioptric optical system, the distance from the reflecting mirror to the wafer becomes insufficient because of the magnification of less than 1. To avoid this, the number of lenses used in the objective lens must be reduced, which causes the optical system to darken. Even if a high NA is achieved, many lens components must be inserted in a limited path and, as a result, the working distance (WD) between the last lens surface of the objective lens and the wafer becomes insufficient.
Another problem in the conventional catadioptric optical system is that the optical axis must be decentered in the middle of the optical path using a decentering lens system, and that precise adjustment of the degree of decentering is very difficult.
The assignee of the present invention proposed a double-focusing optical system with first and second focusing lens systems in a different publication. In this system, the first focusing lens system has a double path lens system comprising a concave mirror and a lens group through which both the incident light to and exit light from the concave mirror pass. The first focusing lens system forms an intermediate image of the first plane (the mask plane), and the second focusing lens system forms that intermediate image on the second plane. A reflecting surface is provided so as to guide the light flux from the first focusing lens system onto the second focusing lens system.
This double-focusing optical system can reduce the diameter of the concave mirror, and has a variable NA ratio .sigma. of the illumination-optical system to the projection-optical system with effective diaphragm positions in both systems. The entire optical system is adequately bright, while the working distance (WD) between the wafer and the end face of the object lens is sufficiently long. In addition, the adjustment of the decentering part in the decentering lens system is simplified, and a highly precise optical system is achieved.
Notwithstanding the advantages, this optical system is likely to become large if still higher image quality is demanded. This optical system does not have a symmetric structure and, accordingly, distortion is likely to arise. Especially, higher order distortion can not be corrected only by adjusting the curvature of the refractive lens or the lens space, and the entire system must be enlarged in order to satisfactorily compensate for the distortion.